1. Field of the Invention
The invention relates to a pyrF gene and its use as selection marker gene for an expression system for producing proteins in fungi of the genera Trametes, Coriolus or Polyporus. 
2. The Prior Art
Various prokaryotic and eukaryotic expression systems are known for producing proteins. The application DE-A-19814853 describes in detail the prior art in this regard. DE-A-19814853 itself discloses a process for transforming filamentous fungi from the genera Trametes and Polyporus, with which it is possible to achieve significantly higher production rates for a protein expressed in each case. The application discloses expression vectors which comprise genetic regulatory elements for expression in filamentous fungi of the class Basidiomycetes. On transformation of filamentous fungi of the class Basidiomycetes they permit positive transformants to be selected on the basis of the complementation of an auxotrophic gene defect.
The gene defect disclosed in DE-A-19814853 relates to the pyrG gene. This gene codes for orotidine-5′-phosphate decarboxylase. DE-A-19814853 also discloses strains with a defect in the pyrg gene which are able to grow on minimal medium only in the presence of uridine (uridine auxotrophy). After transformation of these strains with DNA vectors which comprise an intact pyrG gene, the uridine-auxotrophic strains again grow on minimal medium without uridine (uridine prototrophy).
Uridine-auxotrophic strains are isolated in the state of the art (Boeke et al., Methods Enzymol. (1987) 154, 164–175) by treatment with the genotoxic substance 5-fluoroorotic acid (FOA). Uridine-auxotrophic strains generated on treatment with FOA have a genetic defect either in the pyrG gene or in the pyrF gene. The pyrF gene is also called the ura5 gene. It codes for the enzyme orotate phosphoribosyltransferase.
Uridine-auxotrophic Basidiomycetes strains with a defect in the pyrF gene would also be valuable strains for transformation with the aim of producing proteins if the intact pyrF gene from Basidiomycetes were available as selection marker gene for efficient transformation. However, pyrF genes have to date been described only for fungi from the class Ascomycetes such as, for example, Podospora anserina (Gene 53 (1987), 201–209), Kluyveromyces lactis (unpublished, the DNA sequence is deposited in the “Genbank” database under the accession number klj001358.gb_pl) or Yarrowia lipolytica (M. Sanchez et al., Yeast 11 (1995), 425–433). On the other hand, no pyrF genes from filamentous fungi from the class Basidiomycetes such as, for example, of the genera Trametes, Coriolus or Polyporus are known.